Transmission system



M. E. STRIEBY Dec. 14, 1937.

yTRANSMISS ION SYSTEM Filed F'eb. 2l, 1956 zmm ATTORNEY Patented Dec. 14, 1937 Unirse STATE-s APME FFleE 27,102.138 TRANSMISSION SYSTEM ApplicationFebruary V21, 1936, -Serial No. 65,030

. Y 10 Claims. This. invention relates to wave transmission systems and more particularly to electric wave systems in which a wide frequency range Vis transmitted. Y y

"It is a Awell known'phen'omenon that in Vmany transmission systems waves of different frequencies are not propagated at the same velocity.

Thus, in the transmission yof carrier frequency Waves over a long non-loaded lineV the higher frequencies travel at greater velocity than the lower frequencies, this giving rise, if uncorrected, to perceptible and in vsome cases serious distortion of the signals. Means have been devised heretofore for `differently retarding waves of different frequencies transmitted over a conductingsystem so as to equalize their times of transmission ancl to permit Waves of all frequenciesfto -arrive at a given point of the system in their proper time relation.

`In the'transmission of certain types of signals, and particularly, for example, televisionsignals, it is necessarythat distortion of this character, phase distortion as it is known, be kept at a very low value. So stringent is this requirement that `means are needed not only to equalize the initial' `velocities over the frequency vband but also to' compensate forthe variations of velocity that accompany-slight'changes in the characteristics oithe transmitting medium such, for example, as changes in the `resistance and capacitance'of atransmission line with changes in temperature.

VAn object of the present invention is to correctautomatically for variations in the velocity of transmission and soprevent phase distortion in `a transmission system.

A'more particular object is toprovide a variablephase distortion correction system that does not require a pilot wire circuit or other physical Vtransmission medium paralleling Vthe signal transmission circuit or medium. Y

In another and specic aspect, an object of the finvention is to provide a system in which phase 'distortion correction is effected underthe Vcon- Y trol of waves that are subjected to velocity Vvariations similar to those of the signals under correction; Y c

YA feature of the present invention resides in `the use of a'comparisonwave that is utilized to Idetect yrelative changes in the velocities `olf r, propagation of the signal waves. Y

:Another and more` specificy feature of the in- !vention is the use of two pilot waves of different ifrequencies, the relative propagation velocities of `-wlfilich vary -in av manner'correlated with the variations lin the phase distortion of the signal waves.

In' accordance with the present invention as embodied in a typical illustrative wire transmission system, either or both of two pilot waves are transmitted over a line that is subject to substan- Y tially the same variation in transmission characteristics as the line carrying the signal waves. In the preferred form, both of the pilot-waves and the signal waves are transmitted over the same line, one pilot wave lying immediately above the signal band and the other immediately below it in the frequency spectrum. At a phase distortioncorrecting point, for example, a repeater orterminal station, means are provided for com-f paring the times of transmission of. the two pilot Waves and for varying the signal phase distortion correction at that point in such sense and magnitude as to equalize the times of transmission of the tWo pilot waves or otherwise to Ybring or phase relationship indicative of equal times of transmission.

The nature of the present invention and other objects and features thereof will appear more fully 4in the following description of a typical` plication of the invention to a high frequency i wire line system;

Fig. 2 shows in greater detail thefmeans employed at aV correcting point; and

Figs. 3, 4A, 4B, and 4C are diagrams illustrative ofthe manner of operation of. the correcting means.

Referring now to Fig. 1, there is represented schematically a `one-way wide-band high frequency transmission system comprising terminal mission line in which signal amplifiers are provided at suitable intervals. The signal waves `the pilot waves into some predetermined time 20V ,40 stations l and 2 and an interconnecting trans-- -applied to the Vline at terminal land received at terminal 2 may comprise, for specic example,

la band of carrier television signals 900Vkilocycles in width and lying between 100 and 1000 kilocycles per second in the frequency spectrum utilized in signaling over the line.VA Y Y Y Y At each line repeater is a signal amplier A,

and at two of them are phase distortion correc-Y justable phase only in phase.

able phase distortion correctors. These are operated by means of the respective automatic regulators indicated in Fig. l. The latter, in turn, are controlled by means of waves generated by sources 3 and d and applied to the line at terminal I through respective filters 5 and 6, these waves being, in the embodiment illustrated, both transmitted over the line to filters l and 8 associated with the automatic regulators.

Assuming that the wave sources 3 and 4 generate waves of constant frequencies f1 and f2,-

respectively, f1 lying immediately below the signal spectrum and ,f2 immediately above it, changes in phase distortion occurring inthe transmission line, being reflected in changes in the relative velocities of propagation of the two waves f1 and f2, may be detected at the repeater stationsand caused to effect a compensating change in the v phase distortion correction. An automatic regulator suitable for performing these functions is shown in Fig. 2.

f Although the pilot waves utilized for operating the regulators have been illustrated as generated by separate sources provided for this specific purpose, this is not an essential feature of the present invention. Transmission systems known heretofore have employed two pilot waves for automatic regulation of attenuation and these same pilot waves may be utilized for effecting the objects of the present invention as indicated in Fig. 1. Again, one or more carrier waves utilized in the carrier terminal may be applied to the transmission line and used as pilot waves. In all of these specific examples, one wave subject to substantially the same velocity variations as signals of one frequency or to velocity variations correlated in known manner therewith is compared with another wave that is subject to velocity variations substantially the same as or correlated with, velocity variations of signals of another frequency, whereby relative changes in the velocity at different frequencies within the signal band can be detected and the resulting phase distortion compensated.

Fig. 2 illustrates a preferred embodiment of the invention and shows in detail the nature'of the variable phase distortion equalizer and its associated control circuits.

Briefly, the wide band of carrier television signals produced at terminal I, together with the two pilot waves, are transmitted over the coaxial conductor line I2 to the repeater station,

where both signals and pilot waves are amplified, passed through one or more sections of an addistortion equalizer and out through leads I4 and I5 to the next section of transmission line or to other circuits of the repeater. The two pilot waves are impressed across respective filters, I'I and I8, which are designed `to present such high impedance to the signal circuit that the energy levels of the signal and pilot waves are not substantially affected; the

`one of lower frequency is converted by a harmonic generator or frequency converter to the respective ampliiiers 2I and 22 of constantoutput.

The waves from the constant output amplifiers 2| and 22, being of the same frequency, differ Variations in their phase relation, arising from variations in the relative velocities of transmission of the pilot Waves in the transmission line, are detected by combining the waves in thermocouple 23 or other suitable detecting means. The voltage developed in the thermocouple is then utilized to control the reversible motor 25 which operates to change Athe number of phase distortion equalizer sections in the signaling circuit of the repeater. With the proper number of equalizer sections in circuit, the waves applied to the thermocouple return to some predetermined phase relation and the adjusting operation ceases until further changes in the transmission characteristics of the line occur.

Referring now more specifically to the pilot wave generating means associated with terminal station yI of Fig. 2, the source I0 is, or may be,

` an oscillator of some constant and relatively low frequency. Its output is applied to a harmonic 'generator HG1 and two harmonics thereof are selected by the respective lters 5 and 6 and applied to the transmission line I2. In the embodiment of the invention chosen for purposes of illustration, the frequency f1 of one pilot wave may be immediately below the lower limit of the signaling band, for example 60 kilocycles per second, and the frequency ,f2 of the other may be immediately above the signaling band, for example, 1020 kilocycles per second. The pilot frequencies may be more closely spaced, if desired, and this Will be particularly advantageous where less than the entire signaling frequency range requires distortion correction or where this frequency range is divided into spaced bands of lesser width for independent correction.

Signal and pilot waves arriving at the repeater station are passed through a preliminary amplifier and through one or more of the tandemconnected phase distortion equalizer sections PS0, PS1, PS2, etc., depending on the position of the contact arm of switch I3, and thence through leads I 4 and another amplifier to output circuit leads I5. The phase delay-frequency characteristics of the equalizer sections are represented in Fig. 3, the ordinate indicating the cumulative phase delay (in units of time) at the output terminals of the respective sections. 'I'hese characteristics are such that when added to the phase delay in the line, the total phase delay or time of transmission is the same at all frequencies in the transmitted band under correction.

It will be apparent from Fig. 3 that with increase of frequency the phasev delay in the transmission is less subject to variation. The upper pilot wave, if of sufliciently high frequency, therefore approximates, and from a practical standpoint frequently may be considered as, a reference wave unchanging in velocity of propagation or, vmore broadly stated, as a comparison source of a frequency perfectly xed relative to the lower pilot wave source.

Filters il and I8 connected across the leads I4 are each adapted to select one of the pilot waves passing through those leads. The Voltage of the pilot wave of lower frequency fi is applied to a harmonic generator HG2 and the harmonic, the seventeenth in the specic embodiment described, equal in frequency to the other pilot frequency fz, is selected by means of filter I9. Following filter lIE? in circuit is a phase shifter 20, the purpose of .which will be explained shortly, and an amplifier regardless of changes in the amplitudes of the respective input waves.

Suitable amplifiers of this type are well known in the art.

j'rhe outputr circuits of the ampiiners 2| and 22 are connected in series with each other and with the heater element of thermocouple 23. The respective -output voltages being constant and equal, `the electromotive force developed in the thermocouple depends only on the phase relation between these two output voltages and varies fromzero when the twovoltages are in opposition with respect to the series heater circuit to a maximum` value when they are in phase. It will be apparent; therefore, that with a progressive change in the relative velocities of transmission of the two pilot'waves the thermocouple output voltage tends to vary periodically between zero and a maximum value. Actually, however, the concurrent introduction of phase distortion equalization precludes so wide a range of variation.

The voltage output of thermocouple 23 is applied to the armature of a biased double-contact relay 24, the contacts of which are so connected in the field circuit of direct current motor 25 as to permit reversible control of its operation. The drive shaft of motor 25 is connected, through a speed reducing mechanism if required, in operable relation with the contact arm of switch I3 in such manner as to permit the contact arm to be driven in either direction across its associated contactor segments. Any other suitable phase difference detector, motor and motor control means may be employed alternatively.

Phase shifter 2, designed to establish a normal phase relation between the two waves applied to the thermocouple 23 may be adjusted while the phase distortion introduced by the transmission system is at some average value requiring that, for example, two equalizer sections be connected in the repeater to compensate it. Preferably, the adjustment is such that under these normal conditions the two waves applied to the thermocouple are 120 degrees out of phase, as indicated in Fig. 4A. The vector resultant that eifects operation of the biased relay is then midway between the minimum value that is approached in Fig. 4B and maximum value approached in Fig. 4C. Relay 24 is so biased that under thenormal conditions representedV in Fig. 4A its armature stands midway between the two associated contacts and themotor is at rest.

Upon an increase of the relative delay of the low frequency pilot wave, the phase relations represented in Fig. 4B may be reached. Thereupon the armature of relay 24 moves to complete the motor eld circuit through one of itsV associated contacts, the motordrives the contact arm of switch I3 clockwise to add additionalV sections of the phase distortion equalizer, until Vthe phase distortion is corrected and the phase relations indicated in Fig. 4A` again obtain. Thereupon the armature of relay 24 assumes its neutral position and the system comes to rest.

`Relay 2li is insensitive enough to provideY a neutral zone, as shown in Fig. 4C, to avoid incessant reversal of the motor when slight uncompensated distortion is present.

Upon a decrease of the relative low frequency delay, a similar series of operations is initiated resulting in the removal of one or more equalizer sections from circuit.V It will be obvious that the three adjusting sections shown in Fig. 2y

are only representative of. a larger number that might actually be required. In some instances, the several adjusting sections may have identical delay-frequency characteristics.

In Fig. 2 leads from lters I'I and I8 are indicated for connection to an automatic transmission: regulator. For details of the transmission regulation system, reference is made to E.v I. Green Patent 1,743,132, January 14, 1930, the disclosure of which is to be deemed incorporated in` this specification.

It is to be understood thatV the systems herein described and illustrated are only specific embodiments of applicants invention, and that other and widely different systems may embody applicant's invention and the several novel features thereof as they are dened in the appended claims. Y

What is claimed is:

1. In the transmission of waves occupying a wide frequency range, the method which-comprises subjecting a pilot wave to varying trans.- mission conditions correlated with those conditions to which said wide band of waves is subject, comparing a characteristic of said pilot wave with a reference Wave, and utilizing relative changes in Said characteristic to modify phase distortion within said wide band of waves.

2. In a system subject to variations in phase distortion, the method which comprises transmitting through said system two pilot waves of different frequenciesI and utilizing the variations in the relative velocities of propagation of said pilot Waves to regulate the phase distortion correction in said system.

3. In combination, means for transmitting a band of waves through a medium from a first point to a second point, and an instrumentality at said second point responsive to changes in relative velocities with which waves of different frequencies areY propagated through said medium.

4. In a transmission system subject to variations in phase distortion, means for transmitting over said system two pilot waves of different frequencies, and means responsive to variations in the relative velocities of propagation of said pilot waves. Y

5. A signaling system comprising a transmission line, means for transmitting thereover signal waves occupying a wide frequency band, means for transmitting over said line two pilot waves, one immediately-above and the other immediately below said wide frequency band, Vphase distortion correcting means at a point in said line, means at said point for selecting said pilot waves and converting them to waves of the same frequency, means for comparing the phases of said last-mentioned waves, and means responsive to said last-mentionedmeans for adjustably controlling vsaid correcting means.

6. A combination for detecting difference in phases of two waves comprising means for fixing the respective amplitudes of the said Waves, means for vectorially combining said waves of flxed amplitude, means responsive to the vectorial resultant wave produced by said last-mentioned means, and phase changing means operative on one of said waves to maintain said resultant wave within prescribed amplitude limits. Y

7. In combination, two sources of waves of the same frequency and of variable phase relation, constant output amplifiers each connected to a respective one of said sources, and means responsive proportionally to the vectorially combined outputs of said amplifiers.

' 8. In combination, a transmission system carrection and attenuation equalization in said system, and means operating under the control of said. pilot waves for controlling said adjusting means.

9. A system comprising a conducting line subject to temperature changes, means for transmitting over said line signals occupying a frequency range of the order of megacycles in width, means for concurrently transmitting over said line two Waves having respective frequencies such thatcthe variations in velocity of propagation of said Waves with variation in temperature are comparable with the maximum and minimum velocity'variations respectively of Waves within said frequency range, means for adjustably compensating for velocity variations throughout said frequency range, and means operated under the control of said two waves for adjustably controlling said compensating means.

l0. In a system for the transmission of signals occupying a wide frequency range, means for transmitting over said system a wave subject to velocity variations similar to those to Which said signals are subject, and response means for comparing a condition of said Wave after transmission over said system with a similar condition of a reference Wave, said response means and said condition being such that the response is indicative of changes in the velocity of propagation of said inst-mentioned wave.

MAURICE E. STRIEBY. 

